Ai Weiwei has accepted a teaching job in Berlin but it is unclear when he will be able to take up the post. Photograph: Peter Parks/AFP/Getty Images

The leading Chinese dissident artist Ai Weiwei has accepted a teaching post at a Berlin university, a month after he was released from detention. But it is uncertain whether the Chinese authorities will allow him to leave because he remains under tight surveillance.

Ai, who has endured what he described as "extreme conditions" and an 81-day detention in his home country, said on Thursday he was happy to take the offer of a professorship in Germany and that he would continue to focus on freedom of expression in his art.

"After 81 days put away, I was happy to accept this invitation," he said by phone. "It's an important position, especially when I'm in such a difficult situation."

Ai will not be able to leave China for at least a year because he is under investigation for alleged tax violations and facing tight restrictions.

"My passport has been taken away," he said. "I am not allowed to leave Beijing. I have to report to the police before I go shopping or to a restaurant or to meet friends. They usually allow me to go, but of course I am followed by plainclothes officers."

His lengthy incarceration sparked an international outcry. Ai was taken away by the authorities at Beijing airport on 3 April. He emerged on 22 June, saying he was not allowed to speak about the conditions of his detention. He had clearly lost weight and is now recuperating. "I have to adjust myself – my body. I was in extreme conditions. I have to spend more time with my family," he said.

Since his release the previously outspoken Ai has concentrated on his art rather than his activism, but he said his focus was unchanged despite his recent hardship.

"My art will never change. It is deep in my bones. But it has made many things clearer. I have been working in the direction of freedom of expression. I think that is most important for my art."

The authorities are continuing to put pressure on Ai and his associates. At a hearing on Thursday officials told his wife, Lu Qing, and other representatives of his design firm, Beijing Fake Culture Development Ltd, that the company had not paid corporate taxes for a decade.

Ai will remain under surveillance and tight restrictions until at least 22 June 2012. His status after that is unclear.

The president of the Berlin University of the Arts, Martin Rennert, is optimistic that the artist will be able to take up a guest professorship. Rennart said on the university's website: "We of course also interpret the acceptance as a positive signal as far as his present situation is concerned and are confident that Ai Weiwei will start working at our university in the near future."

A spokeswoman for the university said the decision to offer Weiwei the professorship was made in April, soon after his arrest.

"We only heard a week or so later that the news had reached him," Claudia Assmann said. "And then three days ago we received a written confirmation that he would accept the professorship, not from him directly but from an intermediary." Assmann said Ai said he would very much like to take up the position and felt very honoured.

"The decision to offer him the post was in part a way of showing support for him as a dissident, which was then given greater impetus by his arrest," she said.

"In December we first had talks about whether we should try and involve Ai Wei Wei, an unbelievably interesting personality, with our university. That was at a point at which, yes he was always very critical of the regime but it was not a question of life and limb, the pressure from the Chinese regime was not so much in the foreground. It was then accelerated by his arrest," Assmann said.

"The process of making the offer to him was very advanced and then, when he was arrested in April we wanted and had to react quickly. The whole thing was given a bit more impetus in order to then also give him a political signal."

Asked whether she thought Ai would be able to take up the post, Assman said that the university was ready for him, but that it remained unclear if the Chinese authorities would permit him to go.

"We are prepared for him to come. It would naturally give us great pleasure. It is now up to the Chinese government when he will actually be allowed to leave the country. But we see it as very positive signal that he is now obviously in a situation that he can officially issue an acceptance of the post. We are very confident that it is completely realistic that some day he will come here and teach," she said.

Ai said he was unclear how long the assignment would last, but it could initially be for three years.

If Ai is allowed to leave he would be the second prominent Chinese artist to move to Berlin after being persecuted at home. Last week the author Liao Yiwu arrived in Germany after smuggling himself out of China. He had been denied an exit visa 17 times.